1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) panel, and more particularly, to a liquid crystal display panel with low flicker.
2. Description of the Prior Art
A thin film transistor display, such as a thin film transistor liquid crystal display (TFT-LCD), utilizes many thin film transistors, in conjunction with other elements such as capacitors and bonding pads, arranged in a matrix as switches for driving liquid crystal molecules to produce brilliant images. The advantages of the TFT-LCD over a conventional CRT monitor include better portability, lower power consumption, and lower radiation. Therefore, the TFT-LCD is widely used in various portable products, such as notebooks, personal data assistants (PDA), electronic toys, etc.
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a schematic diagram of a prior art TFT-LCD. FIG. 2 is an equivalent circuit diagram of the TFT-LCD. The TFT-LCD 10 comprises a lower substrate 12. The lower substrate 12 comprises a pixel array 14, a scanning line driving circuit 16, and a data line driving circuit 18. The pixel array 14 includes a plurality of scanning lines (not shown) and a plurality of data lines (not shown). A plurality of pixels (ex. pixels A, B, C, B′, and C′) is therefore defined by the scanning lines and the data lines. The pixel A, B, and C are located on the same scanning line, while the pixel A, B′ and C′ are located on the same data line.
As shown in FIG. 1, the scanning line driving circuit 16 comprises a plurality of driver IC chips (such as chips 16a, 16b, and 16c), which are directly formed on the lower substrate 12 by utilizing chip-on-glass (COG) technology. Additionally, the driver IC chips are connected to each other by several bus lines 17, which is the so-called wiring on array (WOA) technology.
As shown in FIG. 2, a pixel 20 comprises a liquid crystal cell (LC) and a thin film transistor (TFT). The liquid crystal cell (LC) is made of a pixel electrode, a common counter electrode (CE), and a liquid crystal layer inserted there between. The thin film transistor (TFT) comprises a gate electrode connected to a scanning line GL0, a drain electrode connected to a data line DL0, and a source electrode connected to a pixel electrode of the liquid crystal cell. A parasitic capacitor (GS) is produced since the gate electrode and the source electrode of the thin film transistor (TFT) forms an overlapping region. Additionally, the pixel 20 contains a storage capacitor (SC) connected between the liquid crystal cell and a scanning line GL1. The storage capacitor is used to reduce the voltage variation of the liquid crystal cell due to current leakage and thus help the liquid crystal cell store electric charges.
As shown in FIG. 2, the light passing through the pixels varies with the voltage applied to the liquid crystal cell. By changing the voltage to the liquid crystal cell, the amount of light passing through each pixel can be changed and thus the TFT-LCD can display predetermined images. The voltage applied to the liquid crystal cell is the difference between the voltage of the common counter electrode and the voltage of the pixel electrode. When the thin film transistor is turned off, the pixel electrode has a floating status. If any fluctuations occur in the voltages of electric elements around the pixel electrode, the fluctuations will cause the voltage of the pixel electrode to deviate from its desirable voltage. The deviation of the voltage of the pixel electrode referred to feed-through voltage (VFD), which is represented by:VFD=[CGS/(CLC+CGS)]*ΔVG  (1)
where CLC is the capacitance of the liquid crystal cell (LC), CSC is the capacitance of the storage capacitor (SC), CGS is the capacitance between the source electrode and the gate electrode of the thin film transistor, and ΔVG is the amplitude of a pulse voltage applied to the gate electrode.
In general, adjusting the voltage of the common counter electrode can compensate for the feed-through voltage. However, because the resistance and the capacitance of the scanning line round the falling edge of a pulse voltage applied to the gate electrode, a feed-through voltage of a pixel decreases as the distance between the scanning line driving circuit and the pixel increases. For example, as shown in FIG. 1, feed-through voltage of the pixel A is larger than that of the pixel B, whose feed-through voltage is larger than that of the pixel C (that is, (VFD)A>(VFD)B>(VFD)C where (VFD)A, (VFD)B, and (VFD)C represent feed-through voltages of the pixels A, B, C, respectively). Accordingly, it is difficult to compensate feed-through voltages for all pixels by adjusting the voltage of the common counter electrode. Therefore, it is hard to provide a TFT-LCD without flicker.
Furthermore, the resistances of the bus lines are so large that as a pulse voltage is input into the driver IC chips from the bus lines 17, the input voltages of the driver IC chips are different from one another, which leads to different waveforms of output voltages output from the driver IC chips. For example, as shown in FIG. 3, the waveforms of the output voltages output from the chips 16a, 16b, and 16c are quite different. The voltage difference (ΔVGA) output from the chip 16a is larger than the voltage difference (ΔVGB′) output from the chip 16b, which is larger than the voltage difference (ΔVGC′) output from the chip 16c. Therefore, a feed-through voltage of a pixel will decrease as the distance between the data line driving circuit and the pixel increases. That is, as shown in FIG. 1, feed-through voltage of the pixel A is larger than that of the pixel B′, whose feed-through voltage is larger than that of the pixel C′ (that is, (VFD)A>(VFD)B′>(VFD)C′), which make flicker that reduces display quality of an LCD panel.